1. Field of the Invention
This invention relates generally to spindle motors, and more particularly to a spindle motor which is suitable for driving data carriers such as magnetic discs.
2. Description of the Prior Art
As is well known, spindle motors are often used to drive magnetic data storage discs to write in or read out data by the aid of a magnetic read-write head. However, a problem with such a spindle motor is that it permits outflow, from the interior bearing system, of contaminant fluids, such as fluidized grease particles, which may adversely influence the magnetic disc being driven.
U.S. Pat. No. 4,519,010 to Elsaesser et al discloses a spindle motor which comprises a hub mounted to a rotary shaft for supporting one or more magnetic discs, and a stator for electromagnetically rotating the hub. The rotary shaft is rotatably supported in a stationary bushing by means of a bearing system. One end of the bushing is provided with a permanent magnet ring interposed between a pair of annular pole pieces (FIG. 4 of Elsaesser et al). The pole pieces are smaller in inner diameter than the magnet ring and extends radially inwardly into contact with a sleeve, so that an annular cavity is defined by the permanent magnet ring, the pole pieces and the sleeve. The annular cavity is filled with a ferromagnetic fluid to form a magnetic fluid seal against outflow, from the bearing system, of contaminant particles such as grease particles.
The magnetic fluid seal of Elsaesser et al works satisfactorily to prevent contaminant leakage. However, a difficulty arises in filling a predetermined small amount of the magnetic fluid into the annular cavity. Further, the magnetic fluid seal results in complicated structure and high production cost.